Detecting an “invalid date” Date instance in JavaScript

Here’s how I would do it:

if ( === "[object Date]") {
  // it is a date
  if (isNaN(d)) { // d.getTime() or d.valueOf() will also work
    // date object is not valid
  } else {
    // date object is valid
} else {
  // not a date object

Update [2018-05-31]: If you are not concerned with Date objects from other JS contexts (external windows, frames, or iframes), this simpler form may be preferred:

function isValidDate(d) {
  return d instanceof Date && !isNaN(d);

Update [2021-02-01]: Please note that there is a fundamental difference between “invalid dates” (2013-13-32) and “invalid date objects” (new Date('foo')). This answer does not deal with validating date input, only if a Date instance is valid.

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